Download

5.68: Electric Resistance

These athletes are playing rugby, a game that is similar to American football. The players in red and blue are trying to stop the player in orange and black from running across the field with the ball. They are resisting his forward motion. This example of resistance in rugby is a little like resistance in physics.

Q:
In the rugby analogy to resistance in physics, what do the players on each team represent?

A:
The player on the orange and black team represents a moving electron in an electric current. The players on the red and blue team represent particles of matter through which the current is flowing.

Factors that Affect Resistance

How much resistance a material has depends on several factors: the type of material, its width, its length, and its temperature.

All materials have some resistance, but certain materials resist the flow of electric current more or less than other materials do. Materials such as plastics have high resistance to electric current. They are called electric insulators. Materials such as metals have low resistance to electric current. They are called electric conductors.

A wide wire has less resistance than a narrow wire of the same material. Electricity flowing through a wire is like water flowing through a hose. More water can flow through a wide hose than a narrow hose. In a similar way, more current can flow through a wide wire than a narrow wire.

A longer wire has more resistance than a shorter wire. Current must travel farther through a longer wire, so there are more chances for it to collide with particles of matter.

A cooler wire has less resistance than a warmer wire. Cooler particles have less kinetic energy, so they move more slowly. Therefore, they are less likely to collide with moving electrons in current. Materials called superconductors have virtually no resistance when they are cooled to extremely low temperatures. You can learn more about superconductors at this URL:

Is Resistance Good or Bad?

Resistance can be helpful or just a drain on electrical energy. If the aim is to transmit electric current through a wire from one place to another, then resistance is a drawback. It reduces the amount of electrical energy that is transmitted because some of the current is absorbed by particles of matter. On the other hand, if the aim is to use electricity to produce heat or light, then resistance is useful. When particles of matter absorb electrical energy, they change it to heat or light. For example, when electric current flows through the tungsten wire inside an incandescent light bulb like the one in the
Figure
below
, the tungsten resists the flow of electric charge. It absorbs electrical energy and converts some of it to light and heat.

What’s wrong with this picture? (Hint: How does current get to the light bulb?)

Q:
The tungsten wire inside a light bulb is extremely thin. How does this help it do its job?

A:
The extremely thin wire has more resistance than a wider wire would. This helps the wire resist electric current and change it to light.

Summary

In physics, resistance is opposition to the flow of electric charges that occurs as electric current travels through matter. The SI unit for resistance is the ohm.

All materials have resistance. How much resistance a material has depends on the type of material, its width, its length, and its temperature.

Resistance is a hindrance when a material is being used to transmit electric current. Resistance is helpful when a material is being used to produce heat or light.

Vocabulary

resistance
: Opposition to the flow of electric charges that occurs when electric current travels through matter.